Continuos recirculation of the olefin which has not been reacted in the oxidation of olefins by hydroperoxides, by means of solvent scrubbing

ABSTRACT

Process for the continuous recirculation of the olefin which has not been reacted in the oxidation of olefins by means of hydroperoxide to give oxiranes and is present in the offgas stream formed during the oxidation, which comprises the steps (i) to (iii) (i) separating the olefin from the offgas stream by absorption in a hydrocarbon, (ii) desorbing the olefin from the hydrocarbon, (iii) recirculating the olefin obtained in step (ii) to the oxidation process.

The invention relates to a process for the continuous recirculation ofthe olefin which has not been reacted in the oxidation of olefins bymeans of hydroperoxides to give oxiranes, in which the olefin is firstlyabsorbed from the offgas stream formed in the oxidation in a solventcomprising hydrocarbons, preferably tetradecane, subsequently desorbedtherefrom, possibly freed of aliphatics, and recirculated to theoxidation process. The process can be employed particularlyadvantageously for the recirculation of the propene used in thepreparation of propene oxide. The invention also relates to an apparatusby means of which the process can be carried out.

It is known that in the oxidation of olefins by means of hydroperoxidesto give oxiranes, the selectivity of oxirane formation decreasessignificantly and the level of undesirable secondary reactions increasesas the olefin conversion rises. To be able to achieve a high selectivityof over 95% despite this, these reactions are, especially on anindustrial scale, therefore preferably carried out only to an olefinconversion of about 85-95%.

Isolating the unreacted olefin from the reaction process and thenrecirculating it to the oxidation process is also known.

Thus, a process in which a gas mixture comprising the olefin and oxygenoriginating from the decomposition reaction of the hydrogen peroxideused as hydroperoxide in the oxidation is separated off and the olefinis absorbed from the gas mixture in a liquid absorption medium has beenproposed. In this process, a sufficient amount of an inert gas is addedto the oxygen to prevent formation of flammable gas compositions (EP-B 0719 768 B1). In a preferred embodiment, this process is used forrecovering propene from the reaction of propene with hydrogen peroxideto give propene oxide. The inert gas used is preferably methane and theliquid absorption medium used is a mixture comprising isopropanol andwater. Hydrocarbons such as heptane and octane and also methanol andacetone can also be used, but less advantageously, in this process.

However, a disadvantage of the abovementioned process is that a furthergas, in particular methane, in addition to the offgas stream has to befed into the column used for the absorption. This measure is intended toprevent the formation of explosive mixtures in the gas mixture as aresult of the oxygen content.

The low solubility of the olefin in water-containing isopropanol is alsoa disadvantage in this process. For example, based on the description,it is necessary to use solvent mixtures which have a water content offrom 30.6 mol % to 57.2 mol %. This high water content reduces thesolubility of the olefin in the isopropanol. For this reason, relativelylarge amounts of solvent have to be used in order to be able to recoverthe olefin from the offgas stream by absorption.

It is an object of the present invention to provide an improved processfor recovering the olefin used in the oxidation of olefins to oxiranes,which makes do without the additional introduction of an inert gas intothe absorber column and by means of which more effective recovery of theolefin from the offgas stream can be achieved than is possible in theprocess of the prior art.

We have found that this object is achieved by firstly separating theolefin which has not been reacted in the oxidation of an olefin by meansof hydroperoxide to give the oxirane and is present in the offgas streamfrom the offgas stream by absorption in a hydrocarbon, preferablytetradecane, in an absorption plant, liberating it therefrom bydesorption and recirculating it to the oxidation process.

The present invention accordingly provides a process for the continuousrecirculation of the olefin which has not been reacted in the oxidationof olefins by means of hydroperoxide to give oxiranes and is present inthe offgas stream formed during the oxidation, which comprises the steps(i) to (iii)

-   (i) separating the olefin from the offgas stream by absorption in a    hydrocarbon,-   (ii) desorbing the olefin from the hydrocarbon,-   (iii) recirculating the olefin obtained in step (ii) to the    oxidation process.

The hydrocarbon obtained in step (ii) after desorption of the olefin ispreferably recirculated to step (i).

The oxidation of olefins by means of hydroperoxides to give oxiranes isknown and can be carried out by known methods. Such methods and anindustrial process are described, for example, in WO 00/07965.

To separate off the oxiranes formed in the oxidation from the reactionmixtures, it is possible to use, for example, distillation columns.Here, offgas streams are obtained at the top of the columns. Theseoffgas streams always comprise unreacted olefin and a small amount ofoxygen which originates from the decomposition reaction of thehydroperoxide used. To obtain better regulation of the distillation, itis customary to use inert gases, preferably nitrogen. Since these arelikewise taken off at the top of the columns, the offgas streams furthercomprise these gases. It is therefore no longer necessary to feed anadditional gas into the absorption plant to avoid explosive mixtures inthe process of the present invention.

Furthermore, the hydrocarbons used as absorption medium have anexcellent solvent capacity for olefins, so that the process can becarried out using comparatively small amounts of absorption medium. Thenovel process is therefore extremely advantageous for industrial use.

The term hydrocarbons encompasses aliphatic, cyclic, alicyclic,saturated, unsaturated and aromatic hydrocarbons which may also besubstituted by aliphatic radicals. The hydrocarbons can also be used inthe form of mixtures in the process of the present invention. Thehydrocarbons preferably have more than 10 carbon atoms in the molecule.

Preference is given to using tetradecane as hydrocarbon.

For the present purposes, the term tetradecane encompasses a mixture oflong-chain hydrocarbons of the formula C_(n)H_(2n+2), where n is aninteger from 10 to 20, preferably from 13 to 15. The componenttetradecane of the formula C₁₄H₃₀ should be present in this mixture in aproportion of preferably at least 10% by weight, more preferably atleast 30% by weight, in particular at least 50% by weight, wherein thesum of all components present in the mixture is 100% by weight.Accordingly, it is not necessary for the tetradecane used in the processof the present invention to have a particularly high purity. The mixturedescribed can, for example, be obtained in the refining of crude oil,with the mixture corresponding to the desired fraction being taken off.Further components which can as a result be present in addition to thetetradecane are further saturated hydrocarbons which may also bebranched or unbranched, long-chain, cyclic or alicyclic. Unsaturatedhydrocarbons or aromatic hydrocarbons may also be present in such amixture.

The hydrocarbon or hydrocarbon mixture, in particular tetradecane or ahydrocarbon mixture comprising tetradecane, used for absorptionpreferably has a boiling point of from 200 to 300° C., more preferablyfrom 220 to 270° C. Such a hydrocarbon or hydrocarbon mixture displaysan extremely advantageous solvent capacity for the olefins used inolefin oxidation by means of hydroperoxides, in particular for propene.Tetradecene or hydrocarbon mixtures comprising tetradecene display thisadvantageous solvent capacity to a particular extent.

To carry out the process of the present invention, the offgas streamoriginating from a process for the oxidation of an olefin to an oxirane(epoxidation), which may also be a combination of a plurality of offgasstreams, is then advantageously compressed to a pressure of from 2 to 10bar, preferably from 3 to 6 bar, by means of a compressor, subsequentlycooled to preferably from 5 to 35° C. by means of cold water and fed toan absorption plant for the purpose of separating off the olefin.

Such absorption plants preferably comprise the absorption apparatus anda downstream desorption apparatus. End products from such a plant arealways the gas separated off by selective absorption and the tailgasmixture. In addition, the regenerated absorption medium is obtained, andthis is recirculated to the absorption apparatus.

In the absorption plant, it is possible to use packed columns, traycolumns and bubble columns and in special cases Venturi scrubbers,preferably operating in countercurrent. In the overall plant, theabsorption and desorption apparatuses are connected to form acontinuously operating unit.

The offgas stream from the oxidation process is firstly fed into theabsorption apparatus, preferably in the lower part, while thehydrocarbon is fed into the upper part. This results in pronouncedcountercurrent flow of the streams. Further introduction of additionalinert gas, as is necessary in the process disclosed in the prior art, isnot necessary since the offgas stream already contains components fromthe process, preferably nitrogen, which prevent the formation of anexplosive atmosphere.

The pressure of from 2 to 10 bar, preferably from 3 to 6 bar, prevailingin the absorption apparatus increases the solubility of the solubleconstituents of the offgas stream, in particular the unreacted olefin,in the hydrocarbon, which is preferably tetradecane. The pure, insolublegas constituent then leaves the absorption apparatus at the top of thecolumn. This preferably comprises nitrogen, oxygen and small amounts ofolefin, for example propene if this is used in the oxidation process.

Due to the proportion of inert gases in the offgas stream from theoxidation, the separation occurs outside the range in which explosivemixtures with oxygen can be formed. Additional introduction of furthergases to form nonexplosive mixtures therefore becomes superfluous. Thisinsoluble gas can, for example, be passed to incineration.

The olefin-laden hydrocarbon obtained at the bottom of the column ispassed to the desorption stage. There are two possible ways of carryingout the desorption; these are virtually equivalent and the choicebetween them depends on the availability of a refrigeration plant.

If a refrigeration plant using brine (about −35° C.) is available, thedesorption is preferably carried out in a distillation column at apressure of from 1 to 3 bar. In this case, the olefin is freed ofhydrocarbons in the column and separated off in liquid form via the topof the column, brought to a higher pressure level as liquid by means ofa conventional pump and fed either directly to the reaction stage or toa further purification stage. The hydrocarbon freed of olefin isobtained at the bottom and this is cooled and subsequently recirculatedto the absorption column.

If no refrigeration plant is available, it is useful to carry out thedesorption as a single-stage expansion vaporization, also known as flashevaporation, at a pressure of from 1 to 3 bar and a temperature of from50 to 100° C., preferably from 70 to 90° C. In this case, the olefin isseparated off in gaseous form and cooled in a downstream heat exchanger,for example to about 35° C. or to room temperature. The cooled gasstream is compressed to a pressure of about 11 to 12 bar by means of acompressor, so that it can be liquefied using normal river water ascoolant. Suitable compressors are, for example, piston compressors. Thehydrocarbon remaining after the vaporization step in this case stillcontains small concentrations of the olefin. If propene is used asolefin, the proportion is about 1% by weight. The hydrocarbon is cooledin a heat exchanger and recirculated to the absorption stage. Theliquefied gas stream can then be fed either directly to the reactionstage or to a further purification stage.

In a preferred embodiment of the process of the invention, the olefin isaccordingly absorbed at a pressure of from 3 to 6 bar and a temperatureof from 5 to 35° C. and separated off in liquid form at a pressure offrom 1 to 3 bar at the top of a distillation column or in gaseous format a pressure of from 1 to 3 bar and a temperature of from 70 to 90° C.by means of flash evaporation.

In a further embodiment it is possible to operate the absorption plantin the form of an extraction plant. This is of particular interest whenthe olefin to be separated off cannot be desorbed in gaseous form fromthe hydrocarbon on depressurization of the apparatus, but instead formsa liquid phase. Suitable plants comprise, for example, a countercurrentextraction apparatus provided with downstream rectification columns forworking up the hydrocarbon and the liquid olefin.

The olefin, which is preferably obtained in a purity of at least 95%,can then be returned without further purification steps to the oxidationby the hydroperoxide.

A particularly advantageous aspect of the process of the presentinvention is that the olefin can be recirculated to the oxidationprocess in the amount in which it is separated off from the offgasstream from the oxidation. This makes possible an economically extremelyadvantageous process which can be operated continuously.

In a preferred embodiment of the process of the present invention, theoffgas stream further comprises other components, for example saturatedhydrocarbons, in addition to the olefins. The saturated hydrocarbons mayalready have been present in the feed olefin.

Propene is preferably used as olefin in the oxidation stage. Thispreferably contains propane as saturated hydrocarbon. Preference isgiven to using fractions which comprise propene and propane in a volumeratio of from about 97:3 to 95:5. Such mixtures are also referred to as“chemical grade” propene. Preference is given to using propene of thispurity in the preparation of propene oxide.

In a particularly preferred embodiment of the process of the presentinvention, the offgas stream from the conversion of propene into propeneoxide therefore comprises propene as olefin and propane as saturatedhydrocarbon.

As indicated above, the offgas stream then comprises not only thepropene and propane but also inert gases, in particular nitrogen, and asmall amount of oxygen.

In the process of the present invention, the offgas which has been freedof propene and propane is separated off at the top of the absorptionapparatus and the hydrocarbon, preferably tetradecane, laden withpropene and propane is separated off at the bottom of the column.

In the subsequent desorption step, the mixture of propene and propane isseparated either in liquid or gaseous form from the hydrocarbon at apressure of from about 1 to 3 bar using one of the above-describedembodiments. The hydrocarbon is cooled and recirculated to theabsorption stage.

The resulting stream comprising the low-boiling components propene andpropane can subsequently be separated into the components propene andpropane in a C₃ splitter as described, for example, in Ullmann'sEncyclopedia of Industrial Chemistry, 5^(th) Edition, Volume A22, page214. The separation can be carried out in a column at a pressure of fromabout 15 to 25 bar. The separation can also be carried out usingthermally coupled columns, and these are, for example, operated at apressure of about 15 or 25 bar. The propene is taken off at the top ofthe C₃ splitter configured as a column, and the propane is taken off atthe bottom.

Accordingly, another embodiment of the process of the present inventionprovides for the propene/propane mixture obtained after separation fromthe hydrocarbon to be separated into propene and propane in a C₃splitter.

The propene which has been separated off can then be returned to theoxidation by the hydroperoxide. The propane can be used as energy sourcefor steam generation.

Examples of olefins which can be separated off by means of the processof the present invention from offgas streams which are formed in theoxidation of these olefins by means of hydroperoxide to give thecorresponding oxiranes are the following compounds:

ethene, propene, 1-butene, 2-butene, isobutene, butadiene, pentenes,piperylene, hexenes, hexadienes, heptenes, octenes, diisobutene,trimethylpentene, nonenes, dodecene, tridecene, tetradecenes toeicosenes, tripropene and tetrapropene, polybutadienes, polyisobutenes,isoprenes, terpenes, geraniol, linalool, linalyl acetate,methylenecyclopropane, cyclopentene, cyclohexene, norbomene,cyclo-heptene, vinylcyclohexane, vinyloxirane, vinylcyclohexene,styrene, cyclooctene, cyclooctadiene, vinylnorbomene, indene,tetrahydroindene, methylstyrene, dicyclopentadiene, divinylbezene,cyclododecene, cyclododecatriene, stilbene, diphenylbutadiene, vitaminA, beta-carotene, vinylidene fluoride, allyl halides, crotyl chloride,methallyl chloride, dichlorobutene, allyl alcohol, methallyl alcohol,butenols, butenediols, cyclopentenediols, pentenols, octadienols,tridecenols, unsaturated steroids, ethoxyethene, isoeugenol, anethole,unsaturated carboxylic acids such as acrylic acid, methacrylic acid,crotonic acid, maleic acid, vinylacetic acid, unsaturated fatty acidssuch as oleic acid, linoleic acid, palmitic acid, naturally occurringfats and oils.

Hydroperoxides which can be used for the oxidation are all thehydroperoxides which are known from the prior art and are suitable forreaction with the olefin. Examples of such hydroperoxides are tert-butylhydroperoxide and ethylbenzene hydroperoxide. Hydrogen peroxide can alsobe used as hydroperoxide, for example as an aqueous solution.

The offgas streams can also originate from oxidation processes in whichthe reaction of the olefin with the hydroperoxide is catalyzed, forexample by means of heterogeneous catalysts.

In a particularly preferred embodiment of the process of the presentinvention, propene is separated off from an offgas stream obtained inthe oxidation of propene by means of hydrogen peroxide to give propeneoxide. Tetradecane is preferably used as hydrocarbon for thisseparation.

The invention likewise provides an apparatus for carrying out theprocess of the present invention, which comprises at least one reactorfor preparing the oxirane, at least one absorption and desorption unitfor separating off the olefin and a C₃ splitter.

The flow diagrams shown in FIGS. 1 and 2 indicate how propene can berecovered according to the present invention by means of a tetradecanescrub from an offgas stream formed in the epoxidation (oxidation) ofpropene to propene oxide. FIG. 1 shows the recovery without using arefrigeration plant and FIG. 2 shows the recovery when a refrigerationplant is available.

LIST OF REFERENCE NUMERALS FOR THE FIGURES

FIG. 1: A Offgas from oxidation

-   -   B Absorption unit        -   V Compressor        -   W Heat exchanger        -   K Column        -   T Tetradecane        -   I Inert gases (N₂), O₂    -   C Desorption unit        -   V Compressor        -   E Vessel for flash evaporation        -   W Heat exchanger    -   D C3 splitter        -   P Propane (and high boilers)        -   C3″ “Chemical grade” propene

FIG. 2: A Offgas from oxidation

-   -   B Absorption unit        -   V Compressor        -   W Heat exchanger        -   T Tetradecane        -   K Column        -   I Inert gases (N₂), O₂    -   C Desorption unit        -   S Cooling brine (−35° C.)        -   W Heat exchanger        -   H High boilers        -   K Column    -   D C3 splitter        -   P propane        -   C3″ “Chemical grade” propene

1-7. (canceled)
 8. A process for the continuous recirculation of thepropene which has not been reacted in the oxidation of propene by meansof hydroperoxide to give propene oxide, said propene contains propane assaturated hydrocarbon and is present in the offgas stream formed duringthe oxidation, which comprises the steps (i) to (iii): (i) separatingthe propene and propane from the offgas stream by absorption in ahydrocarbon, (ii) desorbing the propene and propane from thehydrocarbon, wherein the mixture of propene and propane is separated offeither in liquid form in a distillation column at a pressure of from 1to 3 bar or in gaseous form at a pressure of from 1 to 3 bar and atemperature of from 50 to 100° C. in a flash evaporation, (iii)recirculating the propene obtained in step (ii) to the oxidationprocess, wherein the propene/propane mixture obtained after separationfrom the hydrocarbon is separated into propene and propane in a C₃splitter before recirculating the propene to the oxidation process. 9.The process as claimed in claim 8, wherein the hydrocarbon obtainedafter desorption of the olefin in step (ii) is recirculated to step (i).10. The process as claimed in claim 8, wherein the hydrocarbon used istetradecane.
 11. The process as claimed in claim 9, wherein thehydrocarbon used is tetradecane.
 12. The process as claimed in claim 8,wherein the propene is absorbed at a pressure of from 3 to 6 bar and atemperature of from 5 to 35° C.
 13. The process as claimed in claim 9,wherein the propene is absorbed at a pressure of from 3 to 6 bar and atemperature of from 5 to 35° C.
 14. The process as claimed in claim 10,wherein the propene is absorbed at a pressure of from 3 to 6 bar and atemperature of from 5 to 35° C.
 15. The process as claimed in claim 8,wherein the offgas stream comprises inert gases and a small amount ofoxygen.
 16. The process as claimed in claim 9, wherein the offgas streamcomprises inert gases and a small amount of oxygen.
 17. The process asclaimed in claim 15, wherein the offgas stream comprises nitrogen. 18.The process as claimed in claim 16, wherein the offgas stream comprisesnitrogen.
 19. An apparatus for carrying out a process for the continuousrecirulation of the propene which has not been reacted in the oxidationof propene by means of hydroperoxide to give propene oxide, said propenecontains propane as saturated hydrocarbon and is present in the offgasstream formed during the oxidation, wherein the apparatus comprises atleast one reactor for preparing the propene oxide, at least oneabsorption and desorption unit for separating off the propene and a C₃splitter, wherein in the absorption unit propene and propane areseparated off by absorption in a hydrocarbon, wherein in the desorptionunit propene and propane are desorbed from the hydrocarbon and whereinin the C₃ splitter the components propene and propane are separated.